The present invention relates to urethane acrylates (UA) of specific composition, to a process for preparing them and to curable compositions comprising said urethane acrylates for coatings, in particular varnishes and inks, the inks more particularly being inks for inkjet, flexography or screen printing application, for adhesives or for adhesion primers and in particular for application by spray gun, for which applications it is sought to reduce the content of reactive diluents such as multi-functional acrylates (MFA) while at the same time being easy to apply. These coating compositions allow good reactivity and a good compromise of performance qualities, in particular between flexibility, adherence and hardness, while at the same time allowing easy application of these compositions under the application conditions. The acrylates mentioned mean for this invention, unless specifically mentioned otherwise, both acrylates and methacrylates.
Urethane acrylates, in particular urethane acrylate oligomers, are well known for their use in coatings, in particular radiation-curable coatings, more particularly UV-curable coatings, with their application performance qualities of good mechanical toughness being associated with their urethane structure, but at the same time having a limitation in their use due to their high viscosity when compared with other acrylates, especially for certain applications for which a low viscosity is desired, in particular in inkjet printing, application by spray gun and other similar applications.
A known solution for overcoming this problem is to dilute them in a reactive diluent, an acrylate monomer having a lower viscosity, in a sufficient manner so as to have an application formulation which allows, by virtue of its adjusted viscosity, its application in the form of a thin film. However, the need to add reactive diluent up to high levels in order to adjust this viscosity has a negative effect on the compromise of the application performance qualities in particular between flexibility, adherence and hardness, the effect of the diluent (such as MFA) being a reduction in the flexibility and deterioration of the adherence (the capacity to adhere) of the film to the substrate and thus insufficient adhesion. Moreover, even if the curable composition contains 100% solids (i.e. no inert solvent to be evaporated), the content of certain monomers also needs to be limited for the sake of the environment or health concerns. Another solution for overcoming the problem of adjusting the final application viscosity is to perform the application at a temperature above room temperature. However, such a solution is impractical, being neither economical nor environmentally- or health-friendly (emission of monomers), with, what is more, more difficult control of the curing reaction due to the thermal effect and the risk of premature curing of the composition with the need for thermal stabilization of the composition.
Another known solution (see in particular WO 02/061 002) consists in modifying the structure of the urethane acrylate oligomer, in particular by using more flexible hydroxyacrylate structures such as those based on caprolactone giving low-viscosity urethane acrylates. However, this solution again requires improvements in terms of reactivity and of performance qualities in terms of compromise between flexibility and adherence and hardness. The existing products, based on caprolactone, are sparingly reactive under UV and require the addition of MFA. Moreover, they do not satisfy the market demand for increasingly lower viscosities that are required by the latest equipment (inkjet). Thus, the growth of inkjet printing in applications such as flexible packaging and labels calls for levels of mechanical performance in terms of flexibility and toughness that are markedly higher than those hitherto required for this technology. This chemistry based on caprolactone hydroxyacrylate and polyisocyanate, after numerous optimizations, has arrived at its limit with regard to what it can offer to ink formulators. Moreover, the improvement of their toughness by increasing the contents of urethane functions by addition of diols leads to higher viscosities. The very fact of reducing the viscosity of UAs, which is precisely associated with the high cohesion of the urethane groups, leads to loss of the essential properties such as the reactivity, the hardness and the flexibility.